1. Field of the Invention
This invention relates to a method of and a device for controlling a stepping motor of the type having a coil and a rotor that is mechanically coupled to a mechanical load and having a permanent magnet that is magnetically coupled to the coil.
The method is of the kind comprising:
applying a drive pulse to the coil whenever the rotor is required to rotate by one step;
short-circuiting the coil a first time at the end of the drive pulse;
returning the coil to open circuit; and
short-circuiting the coil a second time until the beginning of the next drive pulse.
And the device is of the kind comprising:
first means for causing a drive pulse to be applied to the coil whenever the rotor is required to rotate by one step;
second means for causing the coil to be short-circuited a first time at the end of the drive pulse;
third means for causing the coil to be returned to open circuit after the first short-circuiting operation; and
fourth means for causing the coil to be short-circuited a second time after the circuit is opened.
2. Prior Art
Stepping motors of the above type are known and are to be found, in particular, in most electronic timepieces using a hand display.
In such timepieces, the rotor of the motor usually comprises a permanent bipolar magnet whose magnetization axis is perpendicular to the rotation axis of the rotor. This magnet is magnetically coupled to the coil of the motor via a stator comprising a substantially cylindrical opening in which the rotor revolves. Notches made in the wall of the opening apply to the rotor a positioning torque which tends to hold it or to return it to one or another of two stable equilibrium positions.
Such a motor, not shown as it is well known, is used as an example in the following description.
The control circuits of the motors are arranged to apply to the coil a drive pulse whenever the rotor is required to rotate by one step.
In the simpler cases, the length of the drive pulses is fixed. The amount of electrical energy supplied to the motor during the drive pulses is therefore practically independent of the mechanical load it drives. The length of the drive pulses must be sufficient for the rotor to rotate properly even when the mechanical load it drives is at its highest value. But since the load is only rarely at its highest value, a large proprotion of the electrical energy is wasted. The electrical energy is usually provided, particularly in timepieces, by a supply source of limited capacity. Many devices have thus been proposed for reducing the energy consumption of the motor.
All these devices comprise means for determining, in one way or another, the value of the mechanical load being driven by the rotor, and for adjusting the amount of electrical energy supplied to the motor during the drive pulses to the value of the said load.
This adjustment of the amount of electrical energy being supplied to the motor is usually obtained by altering the duration of the drive pulses.
This duration may be determined directly during each drive pulse, as described for instance in U.S. Pat. No. 4,446,413. In such a case, a circuit measures, during each drive pulse, an electrical magnitude that depends on the mechanical load being driven by the rotor of the motor. This circuit generates a signal that causes the current drive pulse being applied to be interrupted when certain conditions are satisfied, these conditions being defined by the nature of the circuit.
The duration of the drive pulses may also be determined indirectly, as described for instance in U.S. Pat. No. 4,272,837. In such a case, a circuit measures, after the drive pulses, a characteristic electrical magnitude that is dependent on the mechanical load which was being driven by the rotor during the drive pulses. If the result of this measurement satisfies certain conditions which are also determined by the nature of the circuit, this indicates that the rotor did not rotate properly in response to the previous drive pulses, and the measurement circuit generates a signal that causes a change in the duration of the following drive pulses. If necessary, the signal generated by the circuit also causes one or several correction pulses to be applied to the motor in order to cause the rotor to perform the step or steps it did not perform in response to the previous pulses.
Most of the control devices mentioned above are arranged so that the coil of the motor is short-circuited, between the end of each drive pulse and the beginning of the next.
This short-circuiting of the motor's coil is performed, in particular, to prevent the rotor from rotating by more than one step if, for some reason, the electrical energy supplied to the motor during a drive pulse is much greater than required, and to cause, between the drive pulses, an electrical braking torque to be applied to the rotor in response to some inadvertent rotation of the rotor due to, for instance, a shock. This electrical braking torque combines with the positioning torque referred to earlier to hold the rotor in the position it is in.
The duration of the drive pulses generated by the devices described above is usually shorter than the time needed for the rotor to reach an angular position from which the positioning torque has a direction and a value such that it can cause, with no external energy supply, the rotor to rotate to its next position of stable equilibrium.
The angular position described above is referred to as threshold angular position in the following description.
This threshold angular position is not fixed, as it depends on the friction which hinders the rotation of the rotor, and is variable.
Between the end of the drive pulse and the instant when the rotor reaches its threshold angular position, the rotor continues to rotate in response to, in particular, its own kinetic energy and that of the various elements it drives.
Furthermore, the short-circuiting of the coil at the end of the drive pulse enables current to carry on flowing within the coil. The greater part of the magnetic energy that is present in the coil at the end of the drive pulse can thus be converted into mechanical energy which cooperates with the kinetic energy of the rotor and of the elements driven thereby to cause the rotor to rotate towards the threshold angular position. Only part of the magnetic energy is dissipated in the form of heat due to the flow of current through the coil.
However, the current in the coil decreases rapidly after the end of the drive pulse. After reaching zero, this current changes direction and the motor begins to operate as a generator.
The electrical energy it then generates, and which is entirely dissipated in the coil in the form of heat, is attributable solely to the conversion of part of the kinetic energy of the rotor and of the elements driven thereby. The rotor is thus braked, and its kinetic energy must overcome the sum of the positioning torque, of a first resisting torque due to the mutual friction of these mechanical elements and of the friction of their pivots in their bearings, of a second resisting torque due to the magnetic phenomena within the motor's stator, and of the torque caused by the electric braking action.
The change in direction of the current, and hence the beginning of the braking action on the rotor, occurs before the rotor reaches the threshold angular position mentioned above. It is therefore necessary for that part of the kinetic energy which is not converted into electrical energy to be sufficient for the rotor to reach the threshold angular position in spite of the braking action.
In other words, the electrical energy that needs to be supplied to the motor for the rotor to rotate properly is made up of a useful part, which is converted into mechanical energy, and of a part which can be considered useless and which is entirely dissipated in the coil after the current in the latter changes direction in the way described above.
Theoretical calculations confirmed by practical tests have shown that, depending on the type of the motor and on the kind of circuit being used for controlling it, the useless electrical energy mentioned above can amount to 25% of the minimum electrical energy that needs to be supplied to the motor for its rotor to rotate properly.
The known methods of and devices for controlling stepping motors therefore suffer from the drawback of causing a substantial drop in the efficiency of the motor. The energy that is uselessly dissipated in the motor must of course be supplied by the electrical supply source of the device. It follows therefore that for a given capacity and hence a given volume of the source its lifetime is substantially shortened or that for a given lifetime of the source its volume needs to be substantially increased.
U.S. Pat. No. 4,467,255 describes a method of controlling a stepping motor wherein, unlike what has been described above, the motor's coil is put on open circuit for a fixed length of time after the end of each drive pulse and is then short-circuited until the beginning of the following drive pulse. The variation in the voltage that is induced in the coil by the rotor's rotation after the end of the drive pulse is used to determine whether the rotor has properly rotated in response to the previous drive pulses.
The drawback of this method is that the magnetic energy that is present in the coil at the end of the drive pulse cannot be converted into mechanical energy since the coil is put on open circuit at that time.
A modification of this method that enables the above drawback to be partly removed is also described in U.S. Pat. No. 4,467,255. In this modification, the coil of the motor is short-circuited at the end of each drive pulse for a fixed, predetermined length of time, before being put on open circuit for another fixed length of time and then being short-circuited again until the beginning of the following drive pulse.
However, the rate at which the current decreases after the coil is first short-circuited depends not only on the characteristics of the coil, but also on the speed reached by the rotor at the end of the drive pulse and thus on the mechanical load driven by the rotor. This rate of decrease of the current is therefore variable. If the length of time that is set for the first short-circuiting operation is shorter than the time taken by the current to become nil, part of the magnetic energy of the coil is not converted into mechanical energy and is therefore lost. But if the length of time that is set for the first short-circuiting operation is longer than the time taken by the current to become nil, the latter changes sign and causes the useless energy dissipation described above.
Further, in the method disclosed in U.S. Pat. No. 4,467,255, the length of time during which the coil is put on open circuit before being short-circuited again is also fixed.
As the speed of the rotor at the end of the drive pulse and after the latter depends on the mechanical load driven by the rotor, the angular position of the rotor at the instant the coil is again short-circuited is variable.
If this angular position is located before the threshold angular position defined above, the rotor is braked by this short-circuiting operation, and again part of its kinetic energy is uselessly dissipated.
If the angular position of the rotor at the time of the short-circuiting operation is close to its second position of stable equilibrium, it could be that its kinetic energy is sufficient to cause it to go beyond the second position of stable equilibrium and reach the next. In such a case, the rotor thus performs two steps instead of one.
The efficiency and reliability of a motor controlled by the method disclosed in U.S. Pat. No. 4,467,255 are therefore not satisfactory.